The question often comes up concerning the different types of AKs. After all, most of what is for sale out there is the AKM model, yet people commonly, and incorrectly, refer to all AKs as AK-47s. AK-47s are one type of AK, as are AKMs and AK-74s. So let me try to shed some light on this. As always, if any of my info is off or incomplete, please let me know so we can all benefit.
I have collected some of these pics over the years and can't recall all the authors or I would give them credit. I can still do that if anyone can validate the origin of a particular picture.

Let's go back to the very beginning. This is the original AK-47, or experimental Assault Rifle No. 1 (1947).

Competitions were held from late 1947 to early 1948 with samples from Dementiev (KB-P-410), Bulkin (TKB-415) and Kalashnikov (AK-47). The results were inconclusive though with Kalashnikov's design not necessarily the best rifle since it suffered in the accuracy department. Regardless, Kalashnikov's design was reliable and that's what the Russians preferred. An order by the Ministry of Armament dated January 21st, 1948, directed both Izhmash and Izhmekh to assist in supplying parts and assemblies for the new rifle. They issued another order in May of 1948 saying that the main production for the new rifle would be done exclusively by Izhmash starting in January of 1949. While production started in mid-1948, official adoption of the AK-47 did not occur until June 18, 1949 by the directive 2611-1033cc of the Council of Ministers. It was formally named 7.62mm avtomat Kalashnikova, yet simply referred to as AK.

This is the first production style AK-47 with a stamped receiver. Two models were made of this first AK with a Type 1A and a Type 1B receiver. The Type 1B receiver had a stamped steel stock that folder underneath and toward the front of the gun, resting under the lower handguard. The picture below is of a Type 1A (fixed stock) receiver. All AK-47s come in 7.62x39.

Here's a picture of one that is still going!

Here is a picture of a Type 1B

Quality problems with the stamped receivers forced Izhmash to switch to a milled receiver until a solution was available. This created the second type of AK-47 receiver, the Type 2A (fixed stock) and the Type 2B (underfolding stock). The new, milled receiver guns were named Oblegchyonnyj (Lightened) 7.62mm avtomat Kalashnikova (AK). Oddly, in the mean time, the Russians were still making the Type 1 despite the high stamped receiver rejection rate. Here is a Type 2A. Note the longitudinal cut just above the mag well that normally identifies the milled AK-47s. Also notice the socket that is added to the back of the receiver to accept the buttstock.

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